The calcitonin (CT)-producing intrathyroidal C-cells represent a subset of dispersed polypeptide hormone-producing neuroendocrine cells. Clinical and pathologic studies have led to the concept that medullary thyroid carcinomas (MTC) in humans with Type II multiple endocrine adenomatosis and in an experimental rat model, are preceded by prolonged phases of C-cell hyperplasia (CCH) and CT hypersecretion. These studies have had a major impact on the early diagnosis of MTC and its precursor, CCH. The rat model offers a reproducible system to characterize both functionally and morphologically the entire spectrum of C-cell proliferative events and to assess the roles of environmental factors in the development of CCH and MTC. In the proposed study, we will seek to determine if environmental factors, such as chronic hypercalcemia in animals with normal and suppressed thyroid function, alter the natural history of the development of C-cell proliferative lesions. To study secretory responses and interrelationships in the absence of modulating influences which are present in the intact animal, we will develop methods for the isolation and enrichment of normal and hyperplastic C-cells for comparison with cultured rat MTC cells. Experiments utilizing these cells will be directed to determine if there are predictable alterations in the cellular contents of CT, neurotensin (NT) and somatostatin (SRIF), secretory granule morphology, and responses to specific stimulators (calcium and glucagon) and inhibitors (SRIF) of CT secretion in normal, hyperplastic and neoplastic C-cells which may be correlated with similar alterations in vivo. The in vitro systems will also be utilized to assess the roles of CT secretagogues and inhibitors and of thyroid follicular hormones in C-cell proliferation. The proposed studies have the potential for serving as models for the analysis of other members of the dispersed polypeptide hormone-producing neuroendocrine cell systems.